Adaptive encoding that encodes orthogonal coefficients such as DFT (Discrete Fourier Transform) and MDCT (Modified Discrete Cosine Transform) coefficients is known as a method for encoding speech signals and audio signals at low bit rates (for example about 10 to 20 kbits/s). For example, AMR-WB+ (Extended Adaptive Multi-Rate Wideband), which is a standard technique, has the TCX (transform coded excitation) encoding mode in which DFT coefficients are normalized and vector-quantized every 8 samples.
In TwinVQ (Transform domain Weighted Interleave Vector Quantization), all MDCT coefficients are rearranged according to a fixed rule and the resulting collection of samples is combined into vectors and encoded. In some cases of TwinVQ, a method is used in which large components are extracted from the MDCT coefficients, for example, in every pitch period in the time domain, information corresponding to the pitch period in the time domain is encoded, the remaining MDCT coefficient strings after the extraction of the large components in every pitch period in the time domain are rearranged, and the rearranged MDCT coefficient strings are vector-quantized every predetermined number of samples. Examples of references on TwinVQ include Non-patent literatures 1 and 2.
An example of technique to extract samples at regular intervals for encoding is the one disclosed in Patent literature 1.